The invention is located in the field of plasma torches.
Arc plasmas belong to the family of thermic plasmas. They are partially ionised gaseous media, conductive of electricity but by and large electrically neutral, at pressures in the region of atmospheric pressure. They are generated by means of a plasma torch, by passing one or more plasmagene gases through an electric arc which is maintained between two electrodes.
To bring gases to a high temperature and high specific enthalpy, blown arc torches are used. This means that the arc is confined to the inside of the torch containing the two electrodes and it is the high speed jet of high temperature gas (the plasma) which is used in the process.
FIG. 1 shows in a very diagrammatic way the principle of such a torch. A torch of this kind includes two electrodes, an anode 1 and a cathode 3, concentric with each other and providing between them a gas circulation channel 7.
The two electrodes 1, 3 are connected to a high voltage, high frequency (HV-HF) generator and to a direct current generator. They must of necessity be energy cooled (by water circulation) to prevent their fusion.
Initially and by means of the HV-HF generator, an electric arc 8 flashes between the two electrodes (cathode and anode) ionising the gas introduced and making the inter electrode space conductive. The direct current generator may then issue into this space and maintain the arc.
The power supplied to the torch is equal to the product of the strength (which can be regulated) of the voltage established between the anode and the cathode. This voltage is dependent on several parameters such as the type and flow of gas used, but also, to a not insignificant degree, on the wear and tear on the electrodes. The power of the plasma 9 is equal to the power supplied to the torch minus the losses in the cooling water. Wear and tear on the electrodes puts them therefore at a serious disadvantage. It depends on their geometry, their cooling efficiency, their coaxiality, and on the type and purity of the gases.
Equipment allowing an arc 8 plasma 9 to be generated is used for thermal spraying (surface treatment), gas heating or chemical synthesis. The energy supplied to the gas(es) by the electric arc allows them to be heated to temperatures above 10,000 K.
The choice of plasmagene gas or gases is almost unlimited. It is a function of the demands of the process (oxidation, nitridation, high temperature in a reduction medium, etc.). The power range is very extensive, running from a few kilowatts to several megawatts. Very often, the potential operational range is dictated by the type and flow of the plasmagene gases selected.
A torch is therefore often designed for a given application since its technology must be compatible with the choice of plasmagene gas and the desired work power.
Its size, its form and its simplicity can also become important if it is required to work in a cramped or hostile environment.
Torches currently in existence are complex units, including at least about ten parts (excluding seals, screws and fluid connectors). The coaxiality of the electrodes depends on the stack of manufactured parts with acceptable tolerances for the seals.
Replacing one or both electrodes is an operation which has to be performed regularly (in most cases after some ten hours in operation). This operation always requires sub-units to be dismantled/re-assembled and the seals to be changed.
To illustrate this, three examples of known plasma torches will now be briefly described.
A first known torch operates with an air/argon or oxygen/argon mix, its power is about 100 kW. It consists of 15 manufactured parts, 21 seals, 22 screws and 6 fluid connectors. The parts subject to regular wear are the cathode and the anode, an insulation bush and an injection nozzle. Minimum maintenance (changing the anode) is required at less than 100 hours of operation in the best conditions of use.
A second known torch has been developed for the hydropyrolysis of heavy hydrocarbons. The plasmagene gases are argon and hydrogen, which are mixed with methane at the torch output. This torch is similar to a thermal spray gun. It has, excluding the fluid feed connectors and the screws, 10 manufactured parts and 7 O-rings.
As a third example may be cited one of the simplest torches, marketed by the company SULZER METCO. This is the thermal spray gun F4-MB. This type of torch operates conventionally with argon, helium and nitrogen singly or in a mixture. Hydrogen is often added to gain power (increase in peak arc voltage). There are nevertheless 8 manufactured parts, 14 O-rings, 12 screw components and 3 fluid connectors.
Japanese patent application JP 04-249 096 describes a plasma torch wherein, in order to reduce the probability of creating an arc between the anode and the cathode, the plasmagene gases follow a path which allows them to eddy. To this end, a centring device 10a, which is a part placed between the anode and the cathode, has an opening 106, which goes from the top face of the centring device to a lateral face. Another conduit 102 located between the anode and the centring device part 10 allows the gases coming from the conduit 106 to be guided to the bottom of the anode.
Conduits 107 join the outside of the centring device 10 to a central cavity 105 of the latter. This particularity allows an eddying jet of plasmagene gas to be created. A more even wear of the cathode is thus obtained.
The purpose of the torch, according to the invention, is as far as possible to simplify the assembly of the torch itself and, on the other hand, the replacement from time to time of worn out parts. It has been developed in particular for a gas heating application in a pyrolysis gas postcombustion reactor for chlorinated radioactive waste, heavily contaminated by alpha emitters. This reactor is intended to operate in a glove box.
In a hostile environment (radioactive, being compelled to work in a glove box or in the remote manipulator), the work must be simplified as far as possible. The standard exchange of sub-units is often preferable to the dismantling and to the re-assembly of isolated parts in a complex unit. The intervention time is shorter, the reliability of a new and inspected sub-unit is much better than that of a dismantled and re-assembled complex unit.
To this end, the plasma torch according to the invention is designed in two parts, a disposable interchangeable cartridge constituting a plasma generator intended to be inserted into a cartridge connecting and holding structure. The purpose of this cartridge connecting and holding structure is to connect the cartridge to its supplies of plasmagene gas, cooling fluid and electrical currents. This structure comprises to this end first cartridge connection means.
These first means serve as intermediaries for the supplies of electrical currents, water and gas. These supplies are therefore completely dissociated from the plasma cartridge.
The structure comprises second means engaging or not engaging with cartridge fixing means so as to keep the cartridge mechanically connected to the first means of supplying electrical currents, water and gas.
The invention relates to a cartridge generating plasma for a plasma torch, having, centered on an axis AAxe2x80x2, an annular anode comprising a central cavity receiving a cathode centered on AAxe2x80x2, the anode and the cathode providing between them an annular space for producing an arc, plasmagene gas distribution means, the distributed gas circulating in the annular space between the cathode and the anode, anode cooling means, comprising particularly conduits for an anode cooling fluid, these conduits having an inlet and an outlet, assembly means, a cartridge characterized in that the plasmagene gas distribution means in the annular cavity formed between the cathode and the anode comprise conduits formed in a central ring of the anode surrounding the central cavity of the anode, a first end of these conduits emerging in the central cavity of the anode, a centering device of the cathode, this centering device having a cylindrical part having a central cavity passed through by the cathode, and tightened on the cathode, this cylindrical part having a lateral outer surface one part at least of which is tightened in the central cavity of the anode, and having an upper surface, the centering device comprising conduits bringing the upper surface of the centering device into communication with a part of the lateral outer surface of the centering device located within the central cavity of the anode, these conduits having a first end in the lateral outer surface of the centering device and second end in the upper surface of the centering device, the conduits of the centering device being in communication with the conduits of the anode.
In this way the plasmagene gas circuit is made with a single auxiliary part, the centering device, by a simple operation carried out under pressure to push along an axial direction a centering device tightened on the cathode in the axial cavity of the anode. With the tightening of the centering device in the anode and on the cathode assembly of the anode, cathode unit is completed. This assembly of the anode on the cathode constitutes moreover a part of the plasmagene gas distribution circuit. In the preferred embodiment continuity and regularity of gas distribution is ensured given that the relay between the centering device conduits and the gas inflow conduits through the anode is provided by an annular distribution volume. The annular distribution volume is constituted, by a radial groove, which may be located either on the anode, or on the centering device, or again both on the anode and on the centering device. In this way the cartridge according to the invention does not require to supply gas any joint or conduit, other than those made by piercing or machining or molding, in the parts required for the torch to operate. As far as assembly is concerned, using a connecting groove between the centering device conduits and the gas inflow conduits through the anode, simplifies assembly since it is not then necessary to index angularly the anode and the centering device.
The plasmagene gases received in the first ends of the conduits of the centering device are distributed around the cathode, by means of several holes emerging in the upper surface of the upper part of the centering device, i.e. in apertures or in a final gas distribution groove.
In the preferred embodiment, where the cathode is carried by a support comprising anode positioning means, an annular cooling volume provided between an assembler and the anode receives a cooling fluid through a conduit bringing the fluid from an outer surface of the cartridge but preferentially from the anode to this annular volume. The assembler, the annular anode and the support comprise hollow parts in the form of annular grooves and projecting parts in the form of annular rings all centered parallel to the axis AAxe2x80x2, the projecting parts being fitted tightly into the hollow parts. The annular volume seal is obtained by the fact that the outer diameter of each projecting ring has a value slightly greater than that of the groove into which it is fitted. In this way the cartridge according to the invention requires in respect of water supply no joint or conduit other than those made by piercing or machining or molding in the parts necessary for the torch to operate. The assembler or assembler body is so called since apart from its function of forming the annular volume around the anode, in which the cooling fluid passes, it also has a cartridge mechanical assembly function. It contributes to the cathode support and anode assembly.
In the preferred embodiment, which will be described below, the assembler is a part made of an electrically insulating material comprising a coaxial lower ring and upper ring. The lower ring is fixed in a support groove, the upper ring is fixed in an anode groove. This anode groove is peripheral to an anode ring. This anode ring houses the central cavity of the anode. In this embodiment the inner radial dimensions of the assembler are greater over at least one axially central part than that of the anode ring housing the central cavity. An annular volume for the circulation of an anode cooling fluid is thus provided between this anode ring and the assembler. This volume is in communication with inlet and outlet conduits of the cooling fluid, by means of conduits bored in the anode, the assembler, or again the support.
Other advantages and benefits of the invention will emerge from the description of a preferred embodiment and of embodiment variants, which will now be described in relation to the appended drawings.